This invention relates generally to antennas and particularly relates to antenna systems characterized by radiated beams having very low sidelobes.
For various applications it is necessary, or at least very advantageous, to be able to radiate a beam having a shaped or predetermined cross-section. Such a beam is capable of illuminating a particular solid angle of space or a specified region on the ground substantially without overlap. This is particularly important for communication purposes where a satellite antenna must illuminate a particular country, state or time zone which may have an irregular shape. This is particularly important to save the frequency spectrum so that different programs may be sent simultaneously into different areas without interference with each other.
In the past, attempts have been made to shape antenna beams. This work was begun in World War II for the purpose of developing microwave antennas for radar applications. This can be effected by multi-element feed array which in turn is used to illuminate a paraboloidal reflector or a lens. Alternatively, the shape of the reflector may be modified to shape the beam by dispersing the rays.
Thus for communication applications, attempts have beem made to illuminate the desired region with a multiplicity of contiguous spot beams. Each spot beam is the main lobe cross-section of a conventional diffraction pattern which is produced by a single feed illuminating a reflector or lens. The desired configuration was then achieved by simply summing the signal voltages of each of the feed elements. However, poor regional configuration is obtained with this technique. In addition, due to the multiple feeds which are displaced from the focal point of the reflector or lens, high sidelobes result. This, of course, means that areas adjacent to the desired zone of coverage are illuminated by substantial power causing highly undesirable interference.
One reason for the poor results is that the radiation patterns of all antennas utilizing lenses and reflectors are seriously degraded when the radiating element is displaced from the focus of the lens or reflector. This is particularly true with the paraboloidal reflector because the sidelobes caused by the diffraction pattern increase substantially in amplitude as the radiating element is displaced from the focal point.
It is accordingly an object of the present invention to provide an antenna array producing a beam of generally Gaussian distribution and substantially without sidelobes.
Another object of the invention is to provide an antenna array of the type which can be relatively simply realized because each radiating element is in co-phase with the others and the powers fed to the elements can be simply determined, whereby the element excitation coefficients are real rather than complex.
A further object of the present invention is to provide an antenna of the type discussed where the spacing between radiating elements and the power excitation can be readily calculated for the desired result of obtaining a Gaussian-shaped beam.
Still another object of the present invention is to provide an antenna array capable of producing a beam of predetermined shape substantially without overlap and with a generally Gaussian distribution.